I used to make this in college while watching scary movies. You can use any flavor jello. Strawberry is the best. I have also given as a gift for those friends who love popcorn.
Pop 2 qts plain popcorn and set aside. In a pan melt butter. stir in marshmallows until softened, but not completely melted. Add Jello then stir until mixture is evenly colored. ( Jello does not have to desolve) drizzle over popcorn and mix.